For continuous fill level determination in containers that contain, for example, liquids or bulk materials, often sensors are used that, according to the transit-time method, measure the transit time of electromagnetic or acoustic waves from the sensor to the filling material surface and back. If the installed location of the sensor relative to the container bottom is known, the sought fill height can be calculated directly from the distance between the sensor and the filling material surface, which distance is determined from the signal transit time by way of the wave propagation speed.
Acoustic waves are generated and emitted, usually in the form of short pulses, by so-called ultrasound fill-level sensors, predominantly in the region of 10 KHz to 100 KHz by means of electromechanical sound transducers. The reflected sound pulses are evaluated either by the same sound transducer or are received by a second sound transducer, which is designed only for receiving, and said sound pulses are evaluated as far as their transit time with reference to the point in time of transmission is concerned.
Electromagnetic waves that can be in a frequency range of between approximately 0.5 GHz and 100 GHz are emitted by the sensor usually by way of antennae and are received back. Such sensors are usually referred to as radar fill-level sensors. Apart from this, devices are known that guide the wave along a waveguide from the sensor to the filling material and back. The reflection of the waves from the filling material surface is based on changes in the propagation impedance for the wave at this location. These devices are also referred to as radar fill-level sensors or, more frequently, as TDR (time domain reflectometry) sensors.
However, it is often difficult to unambiguously distinguish the filling material reflection from all the other echoes.
Furthermore, the shape of the echoes of the filling material is often not the same as a reflection from a flat homogeneous surface would be imaged. This poses the problem of stating, from the found echo of the filling material surface, an unambiguous distance between the sensor and the filling material, and from this of stating a discrete value of the quantity of product contained in the container. This applies less to transit-time measuring in liquids containers, but the problem almost always poses itself in bulk material applications.
However, from the point of view of a user, often an indication of the true fill volume is of interest. In certain cases a knowledge of the topography of the bulk material surface is also useful to the user.
It would be desirable to achieve a spatial resolution of a radar reading, and to improve this resolution to the extent that a three-dimensional image of the reflector (bulk material surface or filling material surface) can be generated.
From DE 10 2005 011 686 A1 a method for measuring the fill level of a medium provided in a container, which method is based on the radar principle, is known in which the measuring signal is emitted to a multitude of different regions, and the reflected components of the measuring signal are received at a multitude of receiving locations. In this way in relation to defined resolution cells in several spatial directions a reflection space to the filling material is to be determined. All the resolution cells together thus provide an image of the three-dimensional topology of the surface of the medium.
From DE 10 2005 011 778 A1 a method for measuring the fill level of a medium provided in a container, which method is based on the radar principle, is known in which the reflected component of the measuring signal is evaluated in a phase-sensitive way, wherein the radar measuring signal is generated by various individual antennae for determining various spatial directions.
From WO 2006/090394 it is known, with the use of an array comprising several sound transmitters or sound receivers, to generate a three-dimensional image of the filling material surface.
Such methods for determining a three-dimensional image of a filling material surface often require relatively great expenditure relating to the mechanics and/or electronics in order to obtain the distance information in various resolution cells. Mechanical movement of the antenna requires servomotors, energy and maintenance. The alternative with group antennae requires considerable expenditure and time relating to signal processing for the phase-sensitive evaluation of the various receiving signals.